The present invention relates to a novel method and apparatus for installing a modular optical shaft encoder. More particularly, the invention provides means for easy centering and gapping of a pattern wheel with respect to a mask and a photodetector in the modular shaft encoder during installation.
Shaft encoders are devices which detect and convert various aspects of the rotation of a rotatable shaft into electronic signals. For example, the encoder could measure the speed of a rotating shaft, or keep track of the number of times that the shaft has rotated.
Two common types of shaft encoders are the self contained encoder and modular encoder. The self contained encoder includes its own shaft upon which the rotatable pattern wheel is mounted. This shaft is coupled to the working shaft. By contrast, the modular encoder's rotatable pattern wheel is mounted directly on the working shaft.
Optical shaft encoders use a photodetector stationary with respect to the shaft and the wheel to detect rotation of the shaft. The photodetector is positioned on an underside of a printed circuit board across a pattern wheel from a light source. Phototransistors are preferred but photodiodes or photovoltaic cells could also be used. Light emitting diodes, LEDs, producing infrared light primarily in the 880 nanometer wavelength are preferred but other LED's or incandescent light sources could be used. The pattern wheel has a transparent disk with an opaque pattern which alternately blocks and transmits the light from the light source to the photodetector as the wheel rotates with the shaft. The mask is positioned between the pattern wheel and the photodetector. It has a pattern which is complimentary to a pattern on the pattern wheel such that the light from the light source is alternately blocked and transmitted as the pattern wheel rotates. Light pulses thereby generated are converted into electronic signals by the photodetector which receives the pulses. The electronic signals contain information about the rotation of the pattern wheel, and hence, the shaft.
The advantages of the modular encoder over the self contained encoders are that the modular encoder is less costly, has fewer working parts and has less rotational mass than the self contained encoder. A lower rotational mass reduces dynamic influence and improves encoder accuracy.
Disadvantages of the modular encoders of the prior art are that they require skill, time and special equipment, such as oscilloscopes and gapping paper, to assemble and precision install. The present invention obviates the need for skill or special tools, and substantially reduces the time required to install a modular optical shaft encoder.